Metal halide lamps are known in the art and are for instance described in EP 0215524 and WO 2006/046175. Such lamps operate under high pressure and have burners or ceramic discharge vessels comprising ionisable gas fillings of for instance NaI (sodium iodide), TlI (thallium iodide), CaI2 (calcium iodide) and/or REIn. REIn refers to rare earth iodides. Characteristic rare earth iodides for metal halide lamps are CeI3, PrI3, HoI3, DyI3 and TmI3.
WO 2005/088673 discloses a metal halide lamp suitable as a projection lamp, for instance as a vehicle headlamp, comprising a discharge vessel surrounded by an outer envelope with clearance and having a ceramic wall which encloses a discharge space filled with a filling comprising an inert gas, such as xenon, and an ionisable salt, wherein in the discharge space two electrodes are arranged whose tips have a mutual interspacing so as to define a discharge path between them, with the special feature that the ionisable salt comprises NaI, TlI, CaI2 and XI3, where X is selected from the group comprising rare earth metals.
WO 2005/088675 discloses a metal halide lamp comprising a discharge vessel surrounded by an outer envelope with clearance and having a ceramic wall which encloses a discharge space filled with a filling comprising an inert gas, such as xenon (Xe), and an ionisable salt, wherein in said discharge space two electrodes are arranged whose tips have a mutual interspacing so as to define a discharge path between them, with the special feature that said ionisable salt comprises NaI, TlI, CaI2 and X-iodide, where X is selected from the group comprising rare earth metals. The ionisable salt may also comprise a halide selected from Mn and In.
Most of the present-day discharge vessels for metal halide lamps have a spherical shape, such as for instance described in DE 20205707, a cylindrical shape, such as for instance described in EP 0215524 or WO 2006/046175, or an extended spherical shape such as for example described in EP 0841687 (U.S. Pat. No. 5,936,351). The latter document describes a ceramic discharge vessel for a high-pressure discharge lamp formed of a cylindrical central part and two hemispherical end pieces, the length of the central part being smaller than or equal to the radius of the end pieces. In this way, the isothermy of the discharge vessel is improved.